1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus.
2. Description of the Related Art
Japanese patent Application Laid-Open No. 2000-077644 relates to a conventional photoelectric conversion apparatus which is used for an AE (Auto Exposure: automatic exposure) sensor or an AF (Auto Focus: automatic focus) sensor to be used in a single-lens reflex camera and employs a phototransistor, for instance. Hereafter, a field effect transistor is referred to as an FET. The photoelectric conversion apparatus has a common source circuit which includes a constant current source and an MOSFET that is driven by the constant current source, and attempts to enhance the photoresponsibility by maintaining a base potential of a phototransistor almost constant, which is determined by a voltage applied between a gate and a source of the MOSFET. When the quantity of incident light changes, the collector current of the phototransistor changes, so the voltage between the base and the emitter of the phototransistor changes. A potential which mainly varies at this time is not the base potential of the phototransistor, but the emitter potential and a potential of the gate of the MOSFET whose source is connected to the emitter. This is because when the photo current varies and the base potential varies, the output of the common source circuit, in other words, the gate potential of the MOSFET whose source is connected to the emitter largely varies, and the emitter potential largely varies at the same time. In this way, the variation of the base potential can be inhibited to be extremely small by feedback applied from the emitter. As a result, the photoelectric conversion apparatus shortens the period of time necessary for charging a capacitance between the base and the collector to achieve improvement in photoresponsibility, even in the case where the phototransistor has a large area and the capacitance between the base and the collector is large.
In addition, Japanese patent Application Laid-Open No. H06-098080 discloses the structure of a photodiode which has been completely depleted.
However, when the area of a phototransistor increases in order to enhance the sensitivity of the pixel of a sensor, the photoresponsibility deteriorates after all by the increase of the capacitance between the base and the collector.
One of the reasons is that when the capacitance between the base and the collector increases, it takes time to charge the capacitance between the base and the collector, even when the variation of the base potential is small.
The variation of the base potential is a value obtained by dividing the variation of the gate potential of the MOSFET whose source is connected to the emitter of the phototransistor by a gain of the common source circuit, and depends on the variate of the incident light, but is typically as small as approximately several mV to a dozen or so mV. However, when the capacitance between the base and the collector is large, it takes time to vary the potential of such a small degree, which influences the photoresponsibility.
Another reason is that when the capacitance between the base and the collector increases, the capacitance between the base and the collector works as a capacitance of a low pass filter at the time when the feedback is applied from the emitter to the base, which delays the feedback.
The above-described capacitance between the base and the collector is mainly formed from a capacitance of a depletion layer in a junction between the base and the collector. There is a technique for completely depleting a photoelectric conversion region, as a general technique for reducing the capacitance of the depletion layer. Even when the photoelectric conversion region has a large area, the capacitance of the depletion layer associated with the photoelectric conversion region can be inhibited to be almost zero by the complete depletion in the photoelectric conversion region. However, because the capacitance of the depletion layer depends on a reverse bias voltage, when the reverse bias voltage to be applied to the photoelectric conversion region varies due to the variation of the photo current in a period while the photoelectric conversion apparatus operates, a parasitic capacitance to be added to the photoelectric conversion region changes according to the operating state. For instance, when the reverse bias voltage varies and falls to a depleting voltage of the photoelectric conversion region or lower, the parasitic capacitance rapidly increases, and the photoresponsibility deteriorates.
For this reason, the present invention is directed at providing a photoelectric conversion apparatus which inhibits the variation of a parasitic capacitance of a photoelectric conversion element according to the operating state to maintain a state in which the parasitic capacitance is small, and thereby can provide a state in which the photoresponsibility is stably satisfactory.